Conventionally, fluorescent luminous tubes, light emitting diodes (LEDs), organic electroluminescent (EL) devices, and so forth are employed as light emitting elements of a print head. A color print head can employ plural kinds of light emitting elements therein. For example, there has been proposed a print head employing an LED as a red light source and fluorescent luminous tubes as a blue light source and a green light source, respectively (see, for example, Japanese Patent Laid-open Application No. 2003-226040: Reference 1). Further, a pulse accumulation method and a pulse weight application method are known to be used for a gradation control of light emitting elements, and there has also been proposed a combination of the two methods (see, for example, Reference 1).
Referring to FIGS. 8 to 10, a conventional print head and a gradation control method therefor will be described.
First, a print head will be schematically explained with reference to FIGS. 8A to 8C.
FIG. 8A shows a configuration of a light recording unit of the print head; FIG. 8B illustrates an array light source in which light emitting elements are disposed in a pattern of an array; and FIG. 8C shows rows of photo-sensitized dots (or photo-exposed dots) formed on a printing paper by being exposed to light emitted from the array light source.
In FIG. 8A, rays of light emitted from an array light source 21R of red light, an array light source 21B of blue light and an array light source 21G of green light are mixed by a dichroic mirror 11 and converged by a lens 12 as a luminous flux LF creating an image on a printing paper 13. The printing paper 13 is moved in a direction indicated by an arrow X1 at a preset speed.
As shown in FIG. 8B, an array light source 21 has m number of light emitting elements dk (k represents an integer in a range from 1 to m) disposed in a pattern of an array. Light emitting elements d1 to dm emit light, while their gradation is controlled based on image data supplied to a driving circuit 22 from a controller 23 such as a CPU. The printing paper 13 is exposed to light emitted from the light emitting elements d1 to dm, whereby photo-sensitized dots (or photo-exposed dots) Pdk (k represents an integer in a range from 1 to m) are formed correspondingly to the light emitting elements d1 to dm during a first cycle of the gradation control, thereby forming a row of photo-sensitized dots Pdl1 on the printing paper 13. Thereafter, the printing paper 13 is moved in a direction of the arrow X1, and a next row of photo-sensitized dots Pdl2 is formed on the paper 13 during a second cycle of the gradation control. In this manner, a single row of photo-sensitized dots is formed for every single cycle of the graduation control.
Now, conventional gradation control methods will be explained with reference to FIGS. 9A and 9B.
FIGS. 9A and 9B describe a pulse accumulation method and a pulse weight-application method, respectively.
FIG. 9A provides a timing chart of data inputs of the driving circuit when executing a gradation control based on image data by employing the pulse accumulation method. The image data is represented by 8 bits.
The gradation is controlled by the light emitting elements of the array light source such that a single row (a single line) of photo-sensitized dots is formed for each cycle of the gradation control Lx. A single cycle of the gradation control Lx is divided into: a light emitting time T1 during which the light emitting elements are operated to emit light; and a non-emission time T2 during which the light emission thereof is stopped. Further, although the light emitting time is defined as a time period for operating the light emitting elements, it can also be regarded as a time period for exposing a printing paper to light or forming photo-sensitized dots on the printing paper by exposing the printing paper to the light.
The light emitting time T1 is divided into 255 sections, thereby performing the gradation control based on 255 steps of the gradation. For example, in case the image data is set to be 0, the driving circuit of the light emitting elements maintains the light emitting elements at “off” while a gradation count increases from 0 to 254, i.e., during the whole light emitting time T1. However, in case the image data is set to be 3, the driving circuit operates the light emitting elements to emit light while the gradation count increases from 0 to 2. Further, in case the image data is set to be 255, the driving circuit operates the light emitting elements to emit light while the gradation count increases from 0 to 254.
FIG. 9B provides a timing chart of data inputs of the driving circuit when executing a gradation control based on image data by employing the pulse weight-application method. The image data is represented by 4 bits.
The image data is expressed by 4 bits of binary numbers, and different light emitting time lengths (weights) are set for each bit. A light emitting time T1 is divided into four time intervals in a manner that the respective time intervals are corresponding to pulses whose widths are equal to 2n (n is an integer in a range from 0 to the number of bits representing the image data), and the respective light emitting time lengths are determined by combinations of the pulses. For example, if the image data is “1”, the driving circuit operates the light emitting elements to emit light during a time period corresponding to a pulse whose width is 20. Further, if the image data is “5”, the driving circuit operates the light emitting elements to emit light during two time periods corresponding to pulses whose widths are 20 and 22, respectively.
Moreover, if the image data is represented by 8 bits, the light emitting time T1 is divided into eight time intervals, and eight different light emitting time lengths are divided in a manner similar to the above-described case.
As described above, the gradation control based on the pulse accumulation method is conducted by dividing the light emitting time T1 into 255 sections if the image data is represented by 8 bits. As a result, the resolution is enhanced, allowing a high image quality. However, since a large number of image data must be transmitted from the control circuit to the driving circuit, the time required for the transmission of the image data is increased, which in turn increases the printing time.
On the other hand, in accordance with the gradation control based on the pulse weight-application method, the number of transmitted image data is only 8 if the image data is represented by 8 bits. Therefore, the time required for the transmission of the image data is shortened, which in turn reduces the printing time. However, the resolution is also reduced, and the quality of printed image is degraded.
To resolve the drawbacks, there has been proposed a method combining the pulse accumulation method and the pulse weight-application method, in which a gradation control is conducted by changing a ratio of the combination of the two methods according to a required printing speed and a required image quality. Hereinafter, a ratio of the pulse weight-application method with respect to the pulse accumulation method in the above-described combination of the two methods will be referred to as a “combination ratio” of the pulse weight-application method.
The resolution of the array light source employed in the print head varies as the number of light emitting elements or light emitting dots per a unit length of a single row of the light emitting elements or the light emitting dots changes depending on the type of the light emitting elements. For example, fluorescent luminous tubes on the market typically have a resolution of 300 dpi (300 dots per an inch), and LEDs on the market typically have a resolution of 600 dpi.
FIGS. 10A and 10B illustrate rows of light emitting elements of two array light sources having resolutions of 300 dpi and 600 dpi, respectively, wherein the number of the light emitting elements in FIG. 10A is m and the number of the light emitting elements in FIG. 10B is twice as many as that in FIG. 10A, i.e., 2m. With regard to a fluorescent luminous tube and an LED employed as the array light sources of the print head, an emitting energy of red light emitted from the fluorescent luminous tube is weak, whereas an emitting energy of red light emitted from the LED is relatively strong. Thus, the LED is generally used for emitting red light, whereas the fluorescent luminous tube is employed for emitting green and/or blue light. Considering the above facts, the fluorescent luminous tube having the resolution of 300 dpi and the LED having the resolution of 600 dpi are often employed together in a single print head.
In the print head employing the two array light sources respectively having resolutions of 300 dpi and 600 dpi, two light emitting elements (e.g., d1 and d2) in FIG. 10B correspond to one light emitting element (e.g., d1) in FIG. 10A. Accordingly, two light emitting elements of the array light source of 600 dpi form two photo-sensitized dots corresponding to one photo-sensitized dot formed by a single light emitting element of the array light source of 300 dpi. That is, two photo-sensitized dots of the resolution of 600 dpi correspond to one photo-sensitized dot of the resolution of 300 dpi.
Since the fluorescent luminous tube and the LED have different resolutions, the advantage of combining the pulse accumulation method and the pulse weight-application method may be difficult to realize depending on the combination method when using the two components in a single print head.
FIG. 10C shows a time period during which image data is transmitted (hereinafter, referred to as “data transmission time”) in accordance with a conventional gradation control method combining the pulse accumulation method and the pulse weight-application method in case where the array light sources have the resolutions of 300 dpi; and FIG. 10D illustrates the same in case where the array light sources have the resolutions of 600 dpi. The image data is represented by 8 bits. Further, the time periods designated as “one cycle of the gradation control” in FIGS. 10C and 10D only show the data transmission times, and the non-emission times are omitted therefrom for simplicity.
As described above, FIG. 10C describes the case of using the array light sources having the resolution of 300 dpi, whereas FIG. 10D depicts the case of using the array light source having the resolution of 600 dpi. In both cases, lower 4 bits of the 8-bit data are controlled by the pulse weight-application method, whereas higher 4 bits thereof are controlled by the pulse accumulation method. During a time period when the pulse weight-application method is applied to the lower four bits, data of 20, 21, 22 and 23 are respectively transmitted, which means that the number of transmission of the image data is equal to 4 (D=4). On the other hand, during a time period when the pulse accumulation method is applied to the higher four bits, data of 24 is transmitted 15 times (D=15). Accordingly, the total number of the transmissions of the image data amounts to 19.
Though the number of transmission of the image data is identical in both cases shown in FIGS. 10C and 10D, the resolution in the case shown in FIG. 10D is two times higher than that shown in FIG. 10C. Thus, the amount of the image data in the case shown in FIG. 10D is doubled compared to that shown in FIG. 10C. Thus, the data transmission time in the case shown in FIG. 10D is also doubled compared to the case shown in FIG. 10C. Accordingly, in order to reduce the data transmission time in the case shown in FIG. 10D thereby being identical to the transmission time in the case shown in FIG. 10C, the data transmission in the case shown in FIG. 10D is needed to be set at a speed two times faster than that in the case shown in FIG. 10C by amplifying a driving frequency of a driving circuit of light emitting elements. In practice, however, there is an upper limit of the performance such as the driving frequency of a driver IC. Therefore, there are occasions where the driving frequency cannot be doubled up. Furthermore, if a high-frequency driver IC is used, the cost increases, and the entire circuit including a data transmission circuit is required to be processed faster. In addition, a measure for suppressing noise is needed for the high-frequency driver IC. Therefore, the entire production cost of the print head increases, thereby making the amplification of the driving frequency of the driving circuit impractical.